Synchronization to upstream bursts

ABSTRACT

The invention relates to a method and apparatus for synchronizing to upstream bursts of frames when a delimiter pattern normally used for the synchronization is a-priory unknown. The method includes identifying in a specific received signaling burst a sequence of pre-defined fixed bits, determining the position and bit pattern of the identified delimiter based on the found position of the fixed bits in the signaling burst, and using the found delimiter pattern to synchronize to following bursts in a burst stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. Provisional PatentApplication No. 61/842,255 filed Jul. 2, 2013, entitled “Method, System,and Device for Frame Synchronization”, which is incorporated herein byreference.

TECHNICAL FIELD

The present invention generally relates to communication networks, andmore particularly relates to a method and apparatus for frame and/orburst synchronization in network connected receivers.

BACKGROUND

A passive optical network (PON) is an optical fiber access technologythat enables a cost effective solution for connecting a large number ofsubscribers. FIG. 1 illustrates an exemplary PON 108 integrated in acommunication network context 100. A plurality of subscriber-sideOptical Network Unit (ONU) devices 105.1 to 105.n are connected to acentral office-side Optical Line Terminal (OLT) device 102 via a passiveOptical Distribution Network (ODN) 107, which typically includes onlypassive components such as optical fibers 103, optical power splitters104, and optical connections 109. The OLT 102 is typically connected toanother network 101, e.g. an Ethernet or the like. The ONUs 105.1 to105.n, which may be generally referred to herein as ONU 105 and whichare connected to the ODN 107 on one side, on the other side aretypically connected with respective subscriber networks or subscriberdevices 106.1 to 106.n, which act as the end receiver and/or source ofthe payload data transmitted in the PON.

In a PON, it is common to define the transmission direction from the OLTto the ONUs as the ‘downstream’ direction, and the transmissiondirection from an ONU to the OLT as the ‘upstream’ direction, and thesedefinitions are used herein. Due to the tree-like topology of a PON, thetransmission modes for downstream and upstream are different. For thedownstream transmission, the OLT broadcasts optical signal to all theONUs in continuous mode (CM), and each particular ONU accepts only thosedownstream frames in the CM stream which headers specify it as thetarget destination of the frame. However, in the upstream channel, ONUsadopt burst mode (BM) transmission wherein each ONU only transmits in atime slot allocated to it by the OLT, so that signals from differentONUs do not overlap at the OLT. Since the timings of the upstream burstsreceived by the OLT from different ONUs are not synchronized, a burstsynchronization procedure has to be performed by the receiving end ofthe OLT or any other PON-connected device attempting to extractinformation contained in the upstream bursts.

One variant of the PON access technology, which is known asGigabit-capable PON (GPON), supports transmission rates in excess of 1Gbit/s and is specified in G.984-series of ITU-T Recommendations. InGPON defined by ITU-T G.984, the upstream bursts generated by the ONUsinclude a 24-bit burst delimiter bit pattern at a pre-determinedposition within the burst. These burst delimiter bit patterns aredefined in messages that the OLT sends to the ONUs at their activation,and are then used by the OLT for the burst synchronization of thereceived upstream bursts, i.e. to determine the start position of theburst.

FIG. 2 illustrates an exemplary method 200 of establishing upstreamburst synchronization in a GPON system, which may be performed by aburst synchronizer function, which may be implemented within an OLT or atest instrument. As shown in FIG. 2, the synchronizer function firstreceives a bit sequence 201 recovered from the received optical burstsignal as known in the art. When the received bit sequence becomes 24bits or larger, the synchronizer function performs a pattern matchingoperation 202 starting with first 24-bits received, to detect whether aportion of the bit sequence matches the 24-bit upstream burst delimiterpattern predetermined as a PON system parameter.

If the matching failed, i.e. the delimiter pattern is not detected, thesynchronizer function shifts the matching position in the bit sequenceby 1-bit, as illustrated by an arrow 202.1. That is, when the currentmatching operation that was performed starting at bit n of the receivedbit sequence did not produce a match, the matching operation 202 is thenrepeated starting at bit n+1 of the received bit sequence, attempting todetect the delimiter pattern again. If the matching succeeds at aparticular alignment of the delimiter pattern and the received bitsequence, as illustrated by an arrow 202.2, the synchronizer functiondetermines the next bit following the delimiter pattern to be thebeginning position of the data portion of the upstream burst, andperforms burst synchronization and data processing 203.

Importantly, the upstream burst delimiter pattern has to be known by thesynchronizer function of the burst receiving device in order to performthe matching operation. In GPON systems defined in ITU-T G.984, thedelimiter pattern to be used for upstream bursts sent from an ONU to theOLT is set by the OLT as an ONU parameter in the activation process ofthe ONU, and in general may be an arbitrary bit pattern of 24-bitslength. In the most common use case the burst synchronization functionis implemented by the OLT when receiving upstream transmission bursts,and is known a priori within the OLT. In the case of a test instrumentthat is connected within the ODN, e.g. inserted at optical connectionpoints 109, the delimiter pattern of the upstream busts is generally notknown a priory. Accordingly, test instruments that are used for testingupstream transmission parameters heretofore had to obtain the delimiterpattern either by user input or by analyzing an Upstream_Overheadmessage as defined in ITU-T G.984, which is transmitted by the OLT indownstream direction in the activation process of the ONU. Therefore,prior art upstream transmission test instruments had to either requirethe user to obtain the delimiter pattern using alternative means andthen input it into the tester, or include circuitry for receiving anddecoding both the upstream and downstream transmission, which increasedtheir cost and implementation complexity. In cases where only upstreamdata contains valuable information, such as in upstream transmissiontesting, it would be desirable to omit downstream receiver facilities ina test instrument.

An object of the present invention is to provide a method and/or devicefor synchronizing to upstream bursts based on information that isobtainable from the upstream transmission without requiring a downstreamtransmission decoding.

SUMMARY

Accordingly, an aspect of the present invention relates to a method forsynchronization to upstream transmission bursts in a network testingdevice, wherein the upstream transmission bursts comprise a delimiterbit sequence that is unknown to the network testing device, the methodcomprising: a) receiving by the network testing device a first upstreamburst signal comprising a first upstream signaling burst, wherein thefirst upstream signaling burst includes the unknown delimiter bitsequence and a sequence of at least partially fixed bits at known fixedbit positions relative to the delimiter bit sequence; b) finding in thereceived burst signal a matching bit pattern that matches at least onepre-defined target bit sequence, wherein the at least one pre-definedtarget bit sequence corresponds to the sequence of at least partiallyfixed bits in the first signaling burst; c) retrieving the delimiter bitpattern from the received first signaling burst based on a positiontherein of the matching bit pattern found in step (b) and the knownposition of the at least partially fixed bits relative to the delimiterbit sequence in the first signaling burst, and saving said delimiter bitpattern in a delimiter memory of the network testing device; and, d)using the saved delimiter bit pattern to synchronize to subsequentlyreceived upstream transmission bursts.

Another aspect of the present invention relates to an optical networktesting device for receiving upstream transmission bursts from adownstream ONU, the network testing device comprising: an optical toelectrical converter for converting a received optical burst signalcomprising an upstream data burst into an electrical data signal,wherein the upstream data burst comprises a delimiter bit sequence; aclock and data recovery unit for converting the electrical data signalinto a received bit sequence representing the upstream data burst; aburst processing logic for determining the position of a delimiter bitsequence in the a received bit sequence; a data processing unit forprocessing data carried by the received upstream burst; and, an outputdevice for outputting processing results. The burst processing logiccomprises: a target bit pattern memory containing one or morepre-defined target bit sequences, the one or more pre-defined target bitsequences representing a sequence of at least partially fixed bits ofone of the upstream transmission bursts; a matching bit pattern finderlogic for finding in the received burst signal a matching bit patternthat matches one of the one or more pre-defined target bit sequences; adelimiter pattern extractor logic for extracting the delimiter bitpattern from the received bit sequence based on the position therein ofthe matching bit sequence found by the matching bit pattern finderlogic; a delimiter memory for saving the delimiter bit pattern; and, aburst synchronization logic for synchronizing subsequently receivedupstream transmission bursts using the delimiter bit pattern saved inthe delimiter memory.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings, which represent preferred embodiments thereof andin which like elements are indicated with like reference numerals, andwherein:

FIG. 1 illustrates a block diagram of an exemplary passive opticalnetwork (PON);

FIG. 2 illustrates a flow chart of a prior art method of upstream bustsynchronization by delimiter patter matching;

FIG. 3 illustrates a schematic representation of a data field structurea GPON upstream burst;

FIG. 4 illustrates a schematic diagram illustrating the conversion by areceiver of an upstream optical burst signal into a received bitsequence;

FIG. 5 illustrates a schematic representation of a data field structureof a first upstream burst generated by an ONU in an ONU activationprocess;

FIG. 6 illustrates an exemplary set of target bit sequencescorresponding to a sequence of at least partially fixed bits in thefirst upstream burst of FIG. 5;

FIG. 7 illustrates a flowchart of a method of finding the delimiter bitsequence in the received first upstream burst using a set ofpre-determined target bit sequences;

FIG. 8 illustrates a schematic functional block diagram of a PON testingdevice;

FIG. 9 illustrates a flowchart of a method of synchronizing to upstreamtransmission bursts in a network testing device without having a-prioriknowledge of the delimiter bit sequence;

FIG. 10 illustrates a flowchart of one embodiment of the method of FIG.9 based on detecting a sequence of at least partially fixed bits in afirst upstream transmission burst generated by an ONU in an ONUactivation process.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular circuits,circuit components, techniques, etc. in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone skilled in the art that the present invention may be practiced inother embodiments that depart from these specific details. In otherinstances, detailed descriptions of well-known methods, devices, andcircuits are omitted so as not to obscure the description of the presentinvention. Furthermore, to facilitate an understanding of the invention,many aspects of the invention are described in terms of sequences ofactions to be performed by functional elements of a tester apparatus fortesting upstream transmission in a bi-directional network such as a PON.It will be recognized that in each of the embodiments, the variousactions including those depicted as blocks in flow-chart illustrationsand block schemes could be performed by specialized circuits, forexample discrete logic gates interconnected to perform a specializedfunction, by computer program instructions being executed by one or moreprocessors, or by a combination of both. Thus, the various aspects ofthe invention may be embodied in many different forms, and all suchforms are contemplated to be within the scope of the invention.

The following definitions are applicable to embodiments of theinvention: the term ‘burst’, as used herein, refers to a transmissiondata structure that is synchronized at reception as a single unit usinga specific bit/symbol sequence that is assumed to be known to thereceiver and is included therein for the purpose of synchronization. Itencompasses the upstream bursts as defined in PON specificationdocuments such as ITU-T G.984, and may encompass data frames or burstsas defined in other transmission systems using asynchronous packettransmission, including but not limited to XG-PON systems as defined inITU-T G.987, and 1G-EPON and 10G-EPON systems defined in IEEE 802.3ahand IEEE 802.3av. Hereinbelow the term ‘frame’ may be usedinterchangeably with the term ‘burst’ and should not be confused withGEM frames or GTC frames as defined in ITU-T G.984.

As used herein, the terms “first”, “second” and so forth may not beintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.

An aspect of the present invention relates to a network testing device,which is exemplified in the present description by an apparatus fortesting upstream transmission in a GPON, hereinafter referred to as aPON tester, and to a method of upstream burst synchronizationimplemented by the tester. One particular aspect of the inventionrelates to a method for obtaining the upstream burst delimiter patternautomatically, i.e. based on upstream transmission without userinteraction, in embodiments where the delimiter pattern is not knowna-priori. The method is based on an observation that an upstreamtransmission typically includes signaling messages that are transmittedusing signaling frames or bursts that include pre-defined fixed bits atpre-defined fixed bit positions. These fixed bits and their positionsmay be defined for example by industry-wide specification documents,such as for example the ITU-T G.984 series of documents for theexemplary case of GPON described herein, and therefore are knowna-priory, and may be used by a receiving device to synchronize to, andto determine the position of the delimiter bit sequence in the frame,and therefore to discover the delimiter bit pattern from the signalingburst itself without knowing it a-priory. Once the delimiter bit patternis discovered from a signaling burst, it can be used to synchronize toconsecutively received bursts generated by the same ONU, which may nothave the known fixed bit positions, or have too few of them for areliable synchronization.

With reference to FIG. 3, there is illustrated by way of example astructure of a GPON upstream burst 300 as defined in ITU-T G.984. Theupstream burst 300 includes a Physical Layer Overhead upstream (PLOu)portion 301 and one or more allocation intervals 302.1 to 302.n. ThePLOu portion 301 is further divided into a preamble section 303,typically used for receiver gain control and clock recovery, a delimitersection 304, typically used for burst synchronization, a bit interleavedparity (BIP) field 305, typically used for error rate calculations, anONU-ID field 306 containing a unique number within the PON that istypically used for ONU identification, and an indication (IND) field307, typically used for status reporting from an ONU to the OLT. Anallocation interval 302 may contain two types of overhead fields, i.e.an upstream physical layer operations, administration and management(PLOAMu) message field 308 and/or an upstream dynamic bandwidth report(DBRu) field 309, and it contains a payload section 310 which generallycontains user data.

The upstream burst 300 may be scrambled, for example using aburst-synchronous scrambling polynomial. GPON systems that comply withITU-T G.984 use the polynomial x7+x6+1. In such systems, the patterngenerated using this polynomial is added modulo two to the upstreamdata. The shift register used to calculate this polynomial is reset toall-ones at the first bit following the delimiter field 304 of the PLOu,and is allowed to run until the last bit of the transmission.

Referring now to FIG. 4, an upstream transmission receiving device 400receives the upstream bursts 300 in the form of an upstream burst signal401, and converts it into a received bit sequence r[n] 403 using aconversion means 402, which implement the optical to electrical (O/E)conversion and data recovery operation as known in the art. As theresult the recovered bit sequence 403 represents the data burst orbursts recovered from the upstream burst signal 401. The upstreamtransmission receiving device 400, of which only the receiving end isillustrated, may be for example the OLT, but more pertinently to thepresent disclosure may be a PON tester instrument for testing theupstream transmission in the PON, which may not have any a-prioryknowledge of the delimiter bit pattern contained in the delimiter field304 of the burst 300.

When the upstream burst 300 is generated by the ONU, the preambleportion 303 has a predetermined length. When receiving the upstreamburst, the receiver device 400, such as the OLT or a PON tester,converts the received burst signal 401 into the bit sequence 403 priorto performing burst synchronization. Due to the conversion process, thelength of the preamble portion 303 in the bit sequence may vary. Due tothe variable length of the preamble portion, the receiving device 400may not know the beginning position of the delimiter portion 304 in thebit sequence 403, and is not capable of determining it simply bycounting a certain number of bits from the first bit received.

In accordance with an aspect of the present invention, the followingmethod may be used to discover the delimiter position in a received bitsequence that is recovered from certain specific upstream burstscarrying pre-defined signaling messages resulting in the existence ofpre-defined bits and bit sequences at fixed bit positions within theburst. Exemplary embodiments of the method described hereinbelow makeuse of a first and optionally of a second upstream burst sent from anONU in the activation process specified in ITU-T G.984; however themethod could be easily adopted to other transmission systems usingspecific bit patterns for burst/frame synchronization and signalingmessages of pre-defined structure that give rise to fixed bit positions.The ITU-T G.984.3 Recommendations document, which is incorporated hereinby reference, specifies that the very first upstream burst that isgenerated by an ONU in the ONU activation process carries aSerial_Number_ONU message. It is also known from the ITU-T G.984Recommendations that in that first ‘signaling’ burst carrying theSerial_Number_ONU message the ONU-ID field 306 is FF′h, and the PLOAMumessage 308 sent in the first allocation interval of that first‘signaling’ burst starts with FF01′h; here and in the following, “ . . .′h” denotes hexadecimal numbers.

Referring now to FIG. 5, there is shown a commented exemplary receivedbit sequence r[n] 501 in the hexadecimal representation including aSerial_Number_ONU PLOAMu message 500. It will be appreciated that eachhexadecimal value corresponds to a specific bit sequence, and thatdefining a specific hexadecimal value singularly defines a correspondingbit sequence. The hexadecimal representation is used hereinbelow merelyas a matter of convenience, as it enables to define a four bit sequenceusing a single letter or numeral. The preamble portion 502, having anunknown number of ‘1 0 1 0’ (hexadecimal A′h) repetitions, is followedby a yet unknown 24-bit long delimiter bit sequence or pattern 503,which is by way of example shown as a hexadecimal “AB6073′h” sequence.Following the delimiter bit sequence 503, there is one-byte sequence 504containing the BIP information, in general these bits may have anarbitrary value. Due to the scrambling, the first ONU-ID field 505 isFB′h, corresponding to unscrambled data FF′h. Following the first ONU-IDfield 505 is the IND field 506 of 8-bits in length which typicallycontains 18′h, corresponding to unscrambled data 00′h, but may containthree other values as described hereinafter. Following the IND field 506is the second ONU-ID field 507 of 8-bits in length which contains AE′h(unscrambled data is FF′h). Following the second ONU-ID field 507 is theMSG-ID field 508 of 8-bits in length which always contains E5′h,corresponding to unscrambled data 01′h. Following the MSG-ID field 508is the vendor-ID field 509 of 32-bits in length, the serial_Number field510 of 32-bits in length, the ability advertisement and random delayfield 511 of 16-bits in length and the CRC field 512 of 8-bits inlength. Depending on the operating conditions of the PON, the IND field506 may alternatively contain 58′h, 78′h, or 38′h depending on the bitsset by the ONU. Thus, the sequence of bits 555 contained in thepre-defined fields 505-508 is at least in part fixed by the ITU GPONspecification; this sequence of bits that is at least partiallypre-defined is referred to herein as the sequence of at least partiallyfixed bits. It will be appreciated that similar bit sequences that areat least partially pre-defined, for example by an industry-standardspecification, may exist in specific frames of other communicationsystems, and that embodiments of the method of the present disclosuremay be easily adopted for such systems.

Referring now to FIG. 6, there are shown five exemplary bit patternsp_(a)[k] 601 to 605 of length K, which the first ‘signaling’ burstcarrying the Serial_Number_ONU PLOAMu message 500 may include at apredefined position within the burst and relative to the delimiter 503,and which are generally referred to herein as the target bit sequencesor patterns 600. As follows from the description hereinabove, the targetbit sequences 600 correspond to the “known”, or ‘fixed’ symbols or bits555 having pre-defined fixed bit positions in the message 500, whichinclude “FB” from the first ONU-ID field 505, “18” or “38” or “58” or“78” or arbitrary “xx” (sequences 601, 602, 603, 604, and 605,respectively,), “AE” from the second ONU-ID field 507, and “E5” from theMSG-ID field 508 as explained above, all hexadecimal. In accordance withan aspect of the present invention, these exemplary target sequences 600may be used as “targets” to find in the received bit sequence 501 thematching pattern of the fixed bits 555, and therefore to determine theexact position of specific pre-defined fields in the received bitsequence, instead of the delimiter sequence that is traditionally usedfor burst synchronization It will be appreciated that in otherimplementation, the fixed bits 555 and the corresponding target bitsequences 600 may be different from those shown in FIG. 6.

Referring now to FIG. 7, there is illustrated one embodiment of a method700 for obtaining the delimiter 503 from a received bit sequence r[n]that is recovered from a specific ‘signaling’ upstream burst, whereinsaid burst includes a sequence of pre-defined at least partially fixedbits at pre-defined bit positions within the burst, using target bitpatterns or sequences p_(a)[k] 600 corresponding to the sequence of atleast partially pre-defined fixed bits. By way of example, the receivedbit sequence r[n] may be the exemplary bit sequence r[n] 501 of length Nshown in FIG. 5, and the exemplary matching or target patterns p_(a)[k]600 of length K may be the target bit sequences 601 to 605 shown in FIG.6.

In the shown embodiment, the method utilizes a pattern matchingfunctionality, which may be referred to hereinbelow as the patternmatching logic, and which compares a target bit pattern 600 to aselected subsequence of bits of the same length K from the received bitsequence r[n], and outputs for example the Hamming distance Dtherebetween, i.e. the number of bit positions where the compared bitsequences differ, or just whether a complete match has been detected(D=0) or not. One skilled in the art would appreciate that such patternmatching functionality can be easily realized using either software orhardware logic.

Embodiments of the method 700 will be described hereinbelow withreference to a flowchart of FIG. 7, and also with reference to FIG. 8illustrating a functional block diagram of an exemplary PON tester 5configured to implement the method; here, only those functional blocksthat pertain to the present invention are shown. As illustrated, the PONtester 5 includes an optical to electrical converter 10 that is followedby a clock and data recovery (CDR) unit 20, which is in turn followed bya burst processing logic 31, a data processing logic 80 and an outputdevice 90. Blocks 20-80 may be implemented using one or more digitalprocessors 21. In operation, the PON tester 5 may be connected to thePON 108 of FIG. 1 at a desired location therein so as to receive theupstream burst signal 401 from one or more ONUs 105, for example at oneof the connectors 109 within the ODN 107. The O/E converter 10, whichtypically includes a photodetector, such as a photodiode, and mayinclude a pre-amplifier, converts the received optical burst signal 401into an electrical signal 13, which is then passed through a clock anddata recovery (CDR) unit 20 to recover therefrom a data signal,typically in the form of the received bit signal r[n] 403 or 501.Possible implementations of the O/E converter 10 and the CDR 20, whichtogether form the conversion means 402 shown in FIG. 4, are well knownin the art and are not described herein. It will be appreciated thatlogic blocks 20, 40, 70, and 80 may be implemented using hardware logic,which may be defined for example in an FPGA, or as software logic. Whenimplemented as software logic, the logic blocks 20, 40, 70, and 80represent a set of computer executable instructions that are saved in amemory device that is defined within, or is readable by, the processor21, which may be in this case embodied for example as a digital signalprocessor (DSP).

Referring again to FIG. 8, the burst processing logic 31 includes abuffer memory 30, a delimiter finder logic 40 including a firstbit/symbol pattern matching logic 44 ₁ and a delimiter patternextracting logic 45, a target pattern memory 50, a delimiter memory 60,and a burst synchronization unit 70 which may also include a secondpattern matching logic 44 ₂. One function of the burst processing logic31 is to determine the exact position of an upstream burst in thereceived bit sequence r[n], which may amount to determining the positionof the first bit of the burst, or of a particular pre-defined field ofthe burst, in the received sequence, and which is referred herein as theburst/frame synchronization. For this purpose, the recovered bitsequence r[n], which may for example represent or include the receivedbit sequences 403 or 501, is first saved in the buffer 30. If thedelimiter memory 60 contains a delimiter pattern 66 for the receivedburst, the burst sync unit 70 may search in the recovered bit sequencer[n] for the position of the bit sequence matching the saved delimiterpattern, for example using matching logic 44 and the method 200described hereinabove with reference to FIG. 2. If the search isunsuccessful, or if the delimiter pattern 66 is not known, the delimiterfinder unit 40 searches for the delimiter bits 503 in the received bitsequence r[n] using one or more target bit patterns saved in the targetpattern memory 50, for example by implementing method 700 describedhereinbelow with reference to FIG. 7, or a variant thereof. Once thedelimiter bit sequence 503 is found in the received bit sequence r[n],it is saved in the delimiter memory 60 as the delimiter pattern 66, andmay be used by the burst sync unit 70 to synchronize the device to newlyreceived upstream bursts. Once the exact position of the delimiter 503,and therefore all other bits of the burst in the received bit sequencer[n] is determined, the data processing unit 80 may perform desired dataprocessing on the received bits, for example to access transmissionparameters and/or quality as known in the art, or for other purposes asdesired for a particular application. The results of the processing maybe saved or communicated to the user using the output device 90, whichmay be for example in the form of a display, a network card, anon-volatile memory device, or any other suitable output device.

Turning now back to FIGS. 7 and 5 while continuing to refer to FIG. 8,the delimiter finder logic 40 may implement an embodiment of the method700, which may be described as follows. After starting a new delimiterpattern search pass 710, in a first step 701 the first matching logic 44₁ is reset to an initial position, which may be defined by a matchingposition counter m=1. For example, the matching start position r[m],which is shown in FIG. 5 by way of example at 514, may be set so thatthe first bit r[n=m=1] of the received bit sequence r[n] is aligned withthe first bit p[k=1] of a target pattern p[k] 600. At step 702, a firstmatching pattern p_(a)[k], with a=1, . . . , A, is selected from a setof the target patterns, for example the set of target patterns 601 to605 shown in FIG. 6, and the selected target pattern is loaded into thematching logic 702; here ‘A’ denotes the total number of the matchingtarget patterns. At step 703, the pattern matching logic 44 performs thematching operation, i.e. compares the vector p_(a) =[p_(a)[1], p_(a)[2],. . . , p_(a)[K]] representing the selected target bit pattern 600 withthe vector m=[r[m], r[m+1], . . . , r[m+K]] representing a same-lengthportion of the received bit sequence r[n]. If a match is detected atstep 704, i.e. the position of the matching sequence 555 formed of thepre-defined fixed bits 505-508 in the received bit sequence r[n] 501 isfound, the operation proceeds to step 705, wherein the delimiterextracting logic 45 determines the position of the delimiter portion 503in the received bit sequence based on the known matching position m 514where the match is detected, and the known positions of the fixed bits505-508 relative to the delimiter 503, and copies the found delimiterbit sequence 503 into the delimiter memory 60. In the exemplary case ofthe GPON system defined by ITU-T G.984, if a pattern match is detectedat the matching start position m, the delimiter pattern is obtained at705 by retrieving the 24-bits bit sequence starting at bit position(m−32) of the received bit sequence r[n], i.e. r[n=m−32], accounting forthe 8-bit BIP sequence 504 between the end of the 24-bit delimiter 503and the beginning of the fixed bit sequence 505-508 that is beingsearched for. A copy of the extracted delimiter sequence 503 may bestored in the delimiter memory 60 as the delimiter pattern 66 forfurther usage and/or processed as desired. It will be appreciated thatthe delimiter pattern 66 may be stored in the delimiter memory 60 indifferent forms, including but not limited to as a binary value, i.e. asequence of bits, or as a hexadecimal value, or in any other suitableform.

If a pattern match is not detected at 704 and the current matchingposition m is less than or equal to (N−K), where N is the number of bitsin the bit sequence r[n] and K is the number of bits in the targetsequence p_(a)[k], the matching position m is incremented at step 707and the pattern matching step 703 is performed again. If a pattern matchis not detected at 704 and the current matching position m is greaterthan N−K, at step 708 the algorithm is either aborted with a matchingpattern error 709, or the matching means is reset 701 and anothermatching pattern is selected and loaded in the matching logic 44 at 702,and steps 703-707 of the pattern matching search with varying alignmentof the matching pattern 600 to the received bit sequence r[n] isexecuted again with the new matching pattern 600.

In one embodiment, the pattern matching logic 44 declares a match instep 704 when vectors p_(a) and m match within a certain pre-definedthreshold Hamming distance C, i.e. when the Hamming distance D betweenvectors p_(a) and m does not exceed C. Here, the Hamming distance D isthe number of unequal bit positions in the bit sequences p_(a) and m,and C is the maximum number of unequal bit positions allowed for a matchbetween p_(a) and m. If the Hamming distance D is greater than C, apattern mismatch is declared, and a new target sequence 600 or targetsequence alignment to the received bit sequence is tried. In general Cmay be selected in the range from 0, which corresponds to the exactmatch between the vectors p_(a) and m, to K, which corresponds to knowmatching elements in the vectors p_(a) and m, but is preferably smallerthan K. In the exemplary embodiment of FIG. 5 wherein the targetpatterns 600 are 32 bit long as shown in FIG. 6, the value of C may beselected for example in the range between 1 and 8, for example 3 or 4.

After obtaining the delimiter pattern from a ‘signaling’ burst, forexample as described hereinabove with reference to FIG. 7 and saving acopy 66 of it in memory 60, consecutive upstream bursts received by thePON tester 5 may be synchronized as known in the art using the savedcopy 66 of the delimiter pattern, for example following the method 200of FIG. 2.

In one embodiment of the method, the delimiter finding logic 40 mayfirst execute the matching of different K-bit portions of the firstreceived bit sequence r[n] to each of the pre-defined target bitsequences 600 in order to determine which of the pre-defined target bitsequences 600 matches a best-matched K-bit portion of the first receivedbit sequence r[n] in the greatest number of bits, and then using theidentified best-matching target bit sequence to determine the positionof the matching bit pattern 555, and therefore of the delimiter 503, inthe first frame bit sequence 501.

In one embodiment of the method, the delimiter finding logic 40 maygenerate a confidence level metric 722 for the found delimiter, whichcharacterizes the level of confidence in the found delimiter pattern. Inone embodiment, the confidence metric 722 may be computed based on, oraccount for, the Hamming distance D between the target pattern 600 andthe matching section of the received bit sequence r[n] for which thematching has been declared.

Optionally, in order to increase robustness of the method describedhereinabove, subsequent upstream bursts may be processed to verify thefound delimiter pattern 66, such as for example, but not exclusively,the second (sequentially) upstream burst generated by the ONU in theactivation process specified in ITU-T G.984. From ITU-T G.984 standardit is known that the second upstream message in the ONU activationprocess is a second Serial_Number_ONU message, which has generally thesame structure as shown in FIG. 3 and FIG. 5, but typically containssomewhat different values in fields 505-508 than the firstSerial_Number_ONU message illustrated in FIG. 5. Hereinafter differentmethods which make the delimiter detection more robust and mayoptionally be applied are briefly described. By utilizing theseadditional methods, the confidence level in the found delimiter may beincreased, and the confidence metric 722 updated, if the additionalmethod confirms the correct delimiter.

One additional method that may be implemented by the burst processinglogic 31 is to search for the delimiter pattern 503, as obtained fromthe first Serial_Number_ONU message with the method 700 described above,in a second received bit sequence r2[n] obtained for example from thesecond Serial_Number_ONU message or a subsequent received burst, forexample using method 200 of FIG. 2. If the delimiter pattern 503 isfound in the second received bit sequence r2[n], the confidence levelmetric 722 is increased; if the delimiter pattern is not found in thesecond received bit sequence r2[n], the confidence level metric 722 isdecreased.

A second additional method is based on utilizing a known structure ofthe preamble sequence 502, which includes multiple repetitions of a samebit sequence, in the example shown in FIG. 5 corresponding tohexadecimal value A′h. In this method, the burst processing logic 31 mayperform a search for a repetitive bit pattern between the first bit inthe received bit pattern r[n] 501 and the found delimiter pattern 503,i.e. to verify that the bits immediately prior to the found delimiter503 in the received bit sequence 501 correspond to a known preamble bitpattern. In one embodiment, if a repetitive bit pattern is recognized,the confidence level metric 722 may be increased; if no repetitivepattern is found, the confidence level metric 722 is decreased.

A third additional method that may be implemented by the delimiterfinding logic 40 is to compare the delimiter pattern obtained from thefirst Serial_Number_ONU message 501 with a set of delimiter patterns 66stored from previous delimiter pattern discoveries, or with customizedpatterns that represent commonly used delimiter patterns.

The embodiments described hereinabove are based on the observation thatcertain ‘signaling’ bursts or frames, such as the very first(sequentially) burst generated by an ONU in the ONU activation processspecified by the ITU-T G.984 that carries the first Serial_Number_ONUmessage 501, may include a sequence of at least partially known ‘fixed’bits at pre-defined fixed bit positions relative to the delimiter, suchas the bits in the fields 505-508 of the first Serial_Number_ONU message501. In other implementations or other transmission systems, thosepre-defined fixed bits may be found in a different frame or burst, whichwould typically have a ‘signaling’ function and thus include knownpre-defined command ‘words’ or bit sequences, which may be used tosynchronize to these specific frames or burst, but which may begenerally absent in other bursts or frames in the same communicationdata stream.

Turning now to FIG. 9, an embodiment 900 of the method of the presentdisclosure for synchronization to upstream transmission bursts in anetwork tester may include the following steps.

At step 910, receiving by the network tester a first upstream burstsignal which includes, or corresponds to, a first upstream signalingburst, which is generated by an ONU connected downstream from thenetwork tester and includes an unknown delimiter and a sequence of atleast partially fixed bits at known fixed bit positions relative to thedelimiter, as exemplified by the delimiter bit sequence 503 and thepre-defined bits in fields 505-508 of the upstream burst 501 illustratedin FIG. 5.

At step 920, the received burst signal is searched for a matching bitpattern that matches at least one pre-defined target bit sequence, asexemplified by the target bit sequences 600 illustrated in FIG. 6. Thepre-defined target bit sequence or sequences corresponds to the sequenceof at least partially fixed bits in the first signaling burst.

At step 930, the delimiter bit pattern is retrieved from the receivedfirst signaling burst based on a position therein of the matching bitpattern found at step 920 and the known position of the at leastpartially fixed bits relative to the delimiter bit sequence in the firstsignaling burst, and the found delimiter bit pattern is saved in adelimiter memory of the network tester. The saved delimiter bit patternis then used to synchronize to subsequently received upstreamtransmission bursts at step 940.

As stated hereinabove, the first upstream signaling burst in method 900may be any transmission burst or frame that contains both the yetunknown delimiter 304 or 503 and a sufficiently long, for example atleast 10-bit or preferably at least 20-bit long, sequence of at leastpartially fixed bits at pre-defined fixed bit positions in the burst. Inexemplary embodiments described hereinabove with reference to FIGS. 3,5, and 6, which pertain to a GPON that complies with ITU-T G.984Recommendations, the first upstream signaling burst may be selected tobe the very first (sequentially) burst that is transmitted by adownstream ONU in the ITU-T G.984 defined ONU activation process. Inthis embodiment, the PON tester may be required to receive the veryfirst burst that the ONU sends after it is connected to the network andis powered on.

Turning now to FIG. 10, an embodiment 901 of the method of the presentdisclosure for synchronization to upstream transmission bursts in a PONtester may start with step 906, wherein the ONU activation process isinitiated, for example the ONU is re-started or powered on for the firsttime, after the PON tester is connected to the network upstream from theONU and is set to be ready to receive upstream transmission burst fromthe ONU. The method than proceeds to step 911, wherein the PON testerreceives the first upstream burst that is generated by the ONU in theONU activation process and contains the first ONU activation message,such as the first Serial_Number_ONU message. At step 921, the firstburst signal received by the tester is converted into the first receivedbit sequence r[n] 501, which at step 931 is searched for a matching bitpattern that matches a pre-determined target bit pattern (600)corresponding to a sequence (555) of the at least partially fixed bitsin the first ONU activation message as described hereinabove. At step941, the delimiter bit pattern 503 is retrieved from the received firstbit sequence r[n] 501 based on a position therein of the matching bitpattern found at step 931, and the known position of the at leastpartially fixed bits 555 relative to the delimiter bit sequence 503 inthe first received burst, and the found delimiter bit pattern 503 issaved in a delimiter memory of the tester. The saved delimiter bitpattern is then used to synchronize to subsequently received upstreamtransmission bursts at step 951.

Advantageously, the method of the present invention, embodiments ofwhich are described hereinabove, enables the PON tester to synchronizeto upstream burst, and extract desired data therefrom, automaticallywithout any a-priory knowledge of the delimiter pattern that isconventionally used for burst synchronization, and without the need toreceive and decode the downstream messages sent by the OLT. This enablesto simplify the PON tester and/or the procedure of testing upstreamsignals in a PON.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentinvention. Thus the present invention is capable of many variations indetailed implementation that can be derived from the descriptioncontained herein by a person skilled in the art. All such variations andmodifications are considered to be within the scope and spirit of thepresent invention as defined by the following claims.

We claim:
 1. A method for synchronization to upstream transmissionbursts in a passive optical network (PON) testing device, wherein theupstream transmission bursts comprise a delimiter bit sequence that isunknown to the PON testing device, the method comprising: a) receiving,by the PON testing device, a first upstream burst signal comprising afirst upstream signaling burst, wherein the first upstream signalingburst includes the unknown delimiter bit sequence and a sequence of atleast partially fixed bits at known fixed bit positions relative to thedelimiter bit sequence; b) identifying, in the received burst signal, amatching bit pattern that matches at least one pre-defined target bitsequence, wherein the at least one pre-defined target bit sequencecorresponds to the sequence of at least partially fixed bits in thefirst signaling burst; c) receiving the delimiter bit pattern from thereceived first signaling burst based on a position therein of thematching bit pattern found in step (b) and the known position of the atleast partially fixed bits relative to the delimiter bit sequence in thefirst signaling burst, and saving said delimiter bit pattern in adelimiter memory of the PON testing device; and d) using the saveddelimiter bit pattern to synchronize to subsequently received upstreamtransmission bursts.
 2. The method of claim 1, further comprisingconverting the upstream burst signal received by the PON testing deviceinto a first received bit sequence, and saving said first received bitsequence in a buffer memory of the PON testing device, wherein step (b)comprises finding the matching bit pattern in the first received bitsequence, and wherein step (c) comprises retrieving the delimiter bitpattern from the saved first received bit sequence.
 3. The method ofclaim 2 wherein step (b) comprises finding, in the first received bitsequence, the matching bit pattern that differs from the at least onepre-defined target bit sequence in at most C bit positions, wherein C isa pre-defined integer that is less than K, and K>1 is the number of bitsin the sequence of at least partially fixed bits.
 4. The method of claim3, wherein the at least one pre-defined target bit sequence comprises aset of pre-defined target bit sequences of length K each, and whereinstep (b) comprises b1) matching K-bit portions of the first received bitsequence to one of the pre-defined target bit sequences to find thematching bit pattern; and, b2) if the matching bit pattern is not foundin (b1), matching K-bit portions of the first received bit sequence to anext target bit sequence selected from the pre-defined target bitsequences until the matching bit pattern is found, or each of thepre-defined target bit sequences is tried.
 5. The method of claim 3,wherein the at least one pre-defined target bit sequence comprises a setof pre-defined target bit sequences of length K each, and wherein step(b) comprises b1) matching K-bit portions of the first received bitsequence to each of the pre-defined target bit sequences; b2)determining which of the pre-defined target bit sequences matches thefirst received bit sequence in the greatest number of bits; and, b3)using the best-matching target bit sequence found in step (b2) todetermine the position of the matching bit pattern in the first receivedbit sequence.
 6. The method of claim 1, further comprising assigning aconfidence level metric to the delimiter bit pattern found in step (c).7. The method of claim 3, further comprising assigning a confidencelevel metric to the delimiter bit pattern found in step (c).
 8. Themethod of claim 7, wherein the confidence level is assigned based on thenumber of bit positions wherein the matching bit pattern found in thereceived first bit sequence differs from the target bit sequence used inthe matching.
 9. The method of claim 6, further comprising: e) receivingby the PON testing device a second upstream burst signal comprising asecond upstream burst; f) searching in the received second upstreamburst for a bit pattern that matches the saved delimiter bit pattern;and, g) increasing or decreasing the confidence level metric dependingon whether the bit pattern that matches the saved delimiter bit patternis found in step (f).
 10. The method of claim 6, further comprisingsearching for a repetitive bit pattern in a portion of the receivedfirst upstream signaling burst preceding the delimiter bit pattern foundin step (c).
 11. The method of claim 6, further comprising comparing thedelimiter bit pattern found in step (c) to one or more pre-determineddelimiter bit patterns.
 12. The method of claim 1, wherein the firstupstream signaling burst signal comprises a first upstream messagetransmitted by a downstream optical network unit (ONU) in an ONUactivation process.
 13. The method of claim 12, wherein the firstupstream message comprised in the first upstream signaling burst is afirst Serial_Number_ONU message generated by the downstream ONU in theONU activation process.
 14. The method of claim 12, further comprisinginitiating an activation process of the downstream ONU prior to step(a), and wherein the first upstream burst signal received by the PONtesting device is the very first burst generated by the downstream ONUin the activation process.
 15. A passive optical network (PON) testingdevice for receiving upstream transmission bursts from a downstreamoptical networking unit (ONU), the PON testing device comprising: anoptical to electrical converter for converting a received optical burstsignal comprising an upstream data burst into an electrical data signal,wherein the upstream data burst comprises a delimiter bit sequence; and,a clock and data recovery unit for converting the electrical data signalinto a received bit sequence representing the upstream data burst; aburst processing logic for determining the position of a delimiter bitsequence in the a received bit sequence; a data processing unit forprocessing data carried by the received upstream burst; and, an outputdevice for outputting processing results; wherein the burst processinglogic comprises: a target bit pattern memory containing one or morepre-defined target bit sequences, the one or more pre-defined target bitsequences representing a sequence of at least partially fixed bits ofone of the upstream transmission bursts; a matching bit pattern finderlogic for finding in the received burst signal a matching bit patternthat matches one of the one or more pre-defined target bit sequences; adelimiter pattern extractor logic for extracting the delimiter bitpattern from the received bit sequence based on the position therein ofthe matching bit sequence found by the matching bit pattern finderlogic; a delimiter memory for saving the delimiter bit pattern; and, aburst synchronization logic for synchronizing subsequently receivedupstream transmission bursts using the delimiter bit pattern saved inthe delimiter memory.